Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Semiconductor devices perform a wide range of functions such as analog and digital signal processing, sensors, transmitting and receiving electromagnetic signals, controlling electronic devices, power management, and audio/video signal processing. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, diodes, rectifiers, thyristors, and power metal-oxide-semiconductor field-effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, application specific integrated circuits (ASIC), power conversion, standard logic, amplifiers, clock management, memory, interface circuits, and other signal processing circuits.
A power conversion circuit converts an input voltage (AC or DC) to a second regulated direct current (DC) output voltage. The regulated DC output voltage is typically a different value than the input voltage. A power conversion circuit is used for any electronic device functioning with a DC operating voltage that derives its power from an alternating current (AC) source. For example, a television that plugs into an AC wall socket uses a power converter to convert the AC power to DC operating potential for the electronic and semiconductor components in the television. The power conversion circuit is also commonly used for charging the battery of a mobile device, e.g., cell phone and laptop computer.
The power converter can be implemented in a number of configurations. For example, a fixed frequency flyback, quasi-resonant flyback, and active clamp flyback are types of power converter circuits. Regulatory authorities establish requirements on power converter standby power and light load efficiency. Many power converters operate at a fixed switching frequency during normal or heavy loading. As the load decreased below a predetermined threshold, the switching frequency is decreased in a controlled fashion, i.e., frequency fold-back mode, to reduce the switching losses and improve overall efficiency of the power converter. During foldback, in one mode of operation, the primary inductor current is frozen and the frequency is varied to achieve output voltage regulation, known as pulse frequency modulation (PFM). Furthermore, during foldback, there is a minimum frequency clamp, typically around 25 kHz, to avoid the operation of the power converter in the audible frequency range.
For instance, with a fixed primary inductor current in PFM mode and the frequency clamped at 25 kHz, the power delivered by a flyback power converter in discontinuous conduction mode (DCM) each cycle is fixed. The fixed power delivery (FPD) point is typically designed to be less than 10% of the maximum power. If the load continues to decrease, the power converter enters skip mode of operation to maintain regulation. Skipping cycles is necessary in standby/near-standby conditions to consume power in 10's of milli-watts range and meet various regulations. The selection of the FPD point is important to the power converter design. If the FPD point is too high, then skip mode is activated at high load possibly resulting in audible noise. If the FPD point is too low, then skip mode may not activate and the target standby power consumption cannot be achieved. For ACF or standard flyback power controller designed to operate over wide frequency range, low power consumption at light loads and FPD entry point can be difficult to achieve due to the varying primary inductance over the switching frequency range.
FIG. 1 illustrates a graph of the primary magnetizing inductance variation versus switching frequency. Primary inductance, shown as line 10, decreases non-linearly with increasing switching frequency. FIG. 2 illustrates the output power of the power converter, shown as line 20, increasing with primary inductance, given a fixed primary current at a fixed frequency. Accordingly, the delivered output power at the desired frequency of operation varies depending on primary magnetizing inductance, which varies with operating frequency. If peak primary current is fixed during foldback across all the possible frequencies of operation of the pulse width modulation (PWM) controller, either skip mode will not activate or fail to meet standby power agency requirements, or the controller will stay in skip mode at heavier loads with audible noise. The deviation in energy delivery can lead to problems in frequency fold-back mode and can generate audible noise for the customer selected primary inductance.